A quadrupole mass spectrometer using a quadrupole mass filter in a mass separator for separating ions in accordance with their mass-to-charge ratio has been known as a type of mass spectrometer. FIG. 6 is a schematic configuration diagram of a general quadrupole mass spectrometer.
A sample molecule is ionized in an ion source 1. The generated ions are converged (and simultaneously accelerated in some cases) by an ion transport optical system 2, such as an ion lens, and injected into a longitudinal space of a quadrupole mass filter 3. The quadrupole mass filter 3 is composed of four rod electrodes (only two electrodes are shown in FIG. 6) arranged in parallel around an ion optical axis C. A voltage of ±(U+V·cos ωt) is applied to each of the rod electrodes, in which a direct-current voltage ±U and a radio-frequency voltage ±V·cos ωt are added. In accordance with this application voltage, only an ion or ions having a specific mass selectively pass through the longitudinal space, while the other ions are dispersed along the way. A detector 4 provides electric signals in accordance with the amount of ions which have passed through the quadrupole mass filter 3.
As just described, the mass of the ions which pass through the quadrupole mass filter 3 changes in accordance with the voltage applied to the rod electrodes. Therefore, by varying this application voltage, the mass of the ions that arrive at the detector 4 can be scanned across a given mass range. This is the scan measurement in a quadrupole mass spectrometer. For example, in a gas chromatograph mass spectrometer (GC/MS) and a liquid chromatograph mass spectrometer (LC/MS), sample components injected into the mass spectrometer change as time progresses. In such a case, by repeating the scan measurement, a variety of components which sequentially appear can be almost continuously detected. FIG. 7 is a diagram schematically illustrating the change in the mass of the ions which arrive at the detector 4.
In such a scan measurement, the voltage applied to the rod electrodes is gradually increased from a voltage corresponding to the smallest mass M1, and when the voltage reaches a voltage corresponding to the largest mass M2, the voltage is immediately returned to the voltage corresponding to the smallest mass M1. Since such a rapid change in the voltage inevitably causes an overshoot (undershoot), a waiting time (settling time) is needed for allowing the voltage to stabilize after the change.
For example, Patent Document 1 discloses that it is inevitable to provide a settling time in a selected ion monitoring (SIM) measurement, and this is also true for the scan measurement. Hence, as shown in FIG. 7, a settling time is provided for every mass scan. During this settling time, a mass analysis of a component injected into the ion source 1 is not performed. Therefore, the longer the settling time is, the longer the time interval is between the mass scans, i.e. the longer the cycle of the mass scan is, which decreases the temporal resolution.
In general, when a mass range that a user wants to monitor (M1 through M2 in the example of FIG. 7) is specified in a mass spectrometer, a mass spectrum for the range is created. However, as an internal operation of the spectrometer, a mass scan is performed across a mass range extended above and below the specified mass range by a predetermined width. That is, even when a mass range of M1 through M2 is specified, a mass scan is performed in which M1−ΔM1 is the initiation point of the mass scan and M2+ΔM2 is the end point thereof. This is because it takes time for the first target ion to be ejected from the quadrupole mass filter after it is injected thereinto; during this period of time, an undesired ion or ions which have previously remained inside the mass quadrupole filter 3 reach the detector 4, which impedes an acquisition of an accurate signal intensity. To take an example, in the case where a mass range to be observed is m/z 100 through 1000, a scan is performed across the mass range of m/z 90 through 1010 with a scan margin of m/z 10 both above and below the mass range to be observed.
The time period of such a scan margin for stably performing a measurement, which is provided outside the mass range necessary for creating a mass spectrum, does not substantially contribute to the mass analysis, just like the settling time. Therefore, in order to increase the temporal resolution of an analysis, it is preferable that the scan margin width is also as small as possible.
[Patent Document 1] Japanese Unexamined Patent Application Publication No.